In 1954, the lot across from the William Penn Hotel was for years an open space used for parking. With a capacity of a little under 100 cars, the lot was a sore spot in the midst of the developing city. The land was eventually bought by the Mellon family to be converted into a beautiful plaza for the citizens of Pittsburgh.

Mellon Square Park, dedicated on October 18, 1955 to the memory of Andrew W. and Richard B. Mellon, was built with funds supplied by three Mellon family foundations. The total cost exceeded $4,500,000. Under the park is a multi-level parking garage with a capacity of 900 cars.